There is an ongoing issue that seem to match the symptoms, although they don't explicitly list the "Active users" report.
MO297870, Microsoft 365 suite, Last updated: November 16, 2021 4:50 AM
Estimated start time: November 12, 2021 2:00 AM
Issue type
Advisory
Status
Service degradation
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User impact
Admins may see reporting delays in the Microsoft 365 admin center and Graph API.